In an automatic computing scale system such as, for example, disclosed in U.S. Pat. No. 2,948,466, a label printing unit is used for successively printing a continuous series of labels with different information corresponding to the weight and computed value of food products or other commodities or articles being weighed. The labels are also frequently printed with certain fixed or common information or data such as the name and grade of the particular food product or commodity. U.S. Pat. No. 3,477,992 discloses a typical label printing apparatus or machine wherein an axially arranged set of rotary type wheels are positioned on one side of a label strip and are preset according to the weight and computed value determined by the computer scale system. A single hammer member is positioned on the other side of the label strip, and is actuated in response to movement of a lever which engages a motor driven cam. As shown in U.S. Pat. Nos. 3,447,992 and No. 3,526,189, the labels may be supplied in the form of pressure sensitive labels which are carried by a paper backing strip, or the labels may be cut from a continuous paper label strip which has a coating of heat activated adhesive on the back side.
In such a label printing apparatus or machine, it is highly desirable for each of the label handling and printing operations to be responsive to an independent corresponding electrical control so that operations may be precisely synchronized to obtain a maximum printing speed and so that the machine is completely compatible with the electrical control signals received from the computing scale system. It is also desirable to provide for conveniently and quickly changing the common information or data which is printed on a series of labels and also to avoid the requirement for purging the printing system through one or more cycles after the common information or data has been changed. In addition, it is desirable for the label printing machine to be of simplified construction in order to minimize maintenance and to be formed of modular units so that each unit may be quickly interchanged or replaced by another unit during a service call in the field and thereby avoid interrupting the use of machine for an extended period of time.